This invention relates to an improved velocity probe which performs more accurate measurement of the velocity of a non-uniform gas flow. Specifically, a new probe geometry is disclosed which enables characterization of a flow in three dimensions in large scale applications.
The power generation industry has been developing equipment and procedures for accurately determining the non-uniform flow of a gas. Such techniques are required for measuring the as-installed performance of large mechanical draft fans or for accurately determining the pollutant emissions from power plant stacks. Obtaining these measurements has been difficult in the past, because in many instances the flow fields are highly three-dimensional in nature. Because the conditions required for accurate duct flow measurements are often unavailable in most plant installations, it becomes necessary to perform velocity probe traverse measurements close to fans or devices (elbows, turning vanes) that disturb the flow. Using conventional pitot-static probes in these areas can lead to large errors in the integrated results.
The American Society of Mechanical Engineers Performance Test Code 11 (ASME PTC-11) now requires the use of directional probes which are capable of accurately resolving a velocity vector into its three Cartesian components. In addition, federal regulations recently promulgated by the Environmental Protection Agency now require that velocity traverses required for emissions sampling to determine compliance be conducted with directional probes when the measurement is being conducted within two equivalent duct diameters downstream or one-half duct diameter upstream of a flow disturbance.
The majority of three-component velocity probe geometries were developed for testing of the flow characteristics around small scale aeronautical models. Very little if any emphasis has been placed on performance and durability of the various geometries in harsh environments. Small geometry probes positioned in industrial air and gas flows would easily become plugged by particulate and moisture. Simply scaling up these small geometry probes to provide larger holes can render many of the probes useless due to the rigors of large scale testing. Such rigors include snagging of sensing heads on ductwork access ports and abrupt contact of the probe with the duct walls and interior supports. These hazards and the resulting damage can quickly render the calibration of a very accurate probe useless.
A velocity vector can be defined by its magnitude and two angles. In order to simultaneously measure these quantities, a probe with at least five pressure sensing ports is required. Two of these ports are used to produce a differential pressure which is unique to a first angle within the range of calibration. Two additional ports provide a differential pressure that is calibrated to a second angle. These angles are referred to as yaw and pitch. The final port is used to indicate total pressure. Local static pressure is determined by calibrating the output of one or more of the yaw and pitch sensing ports to the known static pressure.
There are generally two classes of probes used for characterizing three-dimensional flows. Single purpose probes are those in which total pressure, static pressure, and direction of flow are determined by separate devices. For this class of probes, access to the flow is required in two orthogonal planes and four separate measurements are required. Use of this class of probes is not considered to be appropriate for large scale flow work since both access and time are usually limited. The second class of probes consists of the combination probes which combine all the functions of the single purpose probes into one geometry. Combination probes are generally larger and much simpler to use.
Operation of combination probes can take three forms. A first procedure consists of holding the probe head stationary in the flow, thereby holding the sensing head pointed along a line parallel to the assumed direction of flow. Neither vector angle is nulled or balanced, which requires that the probe first be calibrated over a two-dimensional grid of angle combinations. The no-null procedure provides the user with the simplest and quickest technique for carrying out an extensive duct traverse, but calibration and final analysis of the data become much more complicated. Accuracy can also be sacrificed when either the yaw angle or pitch angle becomes large.
To avoid the complexities of the no-null calibration and data reduction, a second operational procedure, the single-null method, requires that the probe always be nulled or balanced in the yaw angle. This method allows calibration of only the pitch pressure differential over a range of pitch angles.
The third operational procedure requires nulling or balancing those pressures which indicate the two angles of the velocity vector by rotating the sensing head relative to both axes. This technique, called the double-null procedure, is not a practical alternative in large applications since duct penetrations allow only rotation of the probe to provide determination of the yaw angle.